Tail rotor

Thetail rotoris a smallerrotormounted vertically or near-vertically at thetailof a traditional single-rotorhelicopter,where it rotates to generate apropeller-like horizontalthrustin the same direction as themain rotor's rotation. The tail rotor's position and distance from the helicopter'scenter of massallow it to develop enough thrustleverageto counter thereactionaltorqueexerted on thefuselageby the spinning of themain rotor.Without the tail rotor or other anti-torque mechanisms (e.g.NOTAR), the helicopter would be constantly spinning in the opposite direction of the main rotor when flying.

Traditional tail rotor of anSikorsky S-61

Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis. Its drive system consists of ashaftpowered from the maintransmissionand agearboxmounted at the end of thetail boom.Thedrive shaftmay consist of one long shaft or a series of shorter shafts connected at both ends with flexiblecouplings,that allow the drive shaft to flex with the tail boom. The gearbox at the end of the tail boom provides an angled drive for the tail rotor and may also include gearing to adjust the output to the optimum rotational speed for the tail rotor, measured inrotations per minute(RPM). On larger helicopters with a tail pylon, intermediate gearboxes are used to transition the tail rotor drive shaft from along the tailboom to the top of the pylon. The tail rotor pylon may also serve as a vertical stabilizingairfoil,to alleviate the power requirement for the tail rotor in forward flight. The tail rotor pylon may also serve to provide limited antitorque within certainairspeedranges, in the event that the tail rotor or itsflight controlsfail. About 10% of the engine power goes to the tail rotor.

Design

The tail rotor system rotates airfoils, small wings calledblades,that vary in pitch in order to vary the amount of thrust they produce. The blades most often utilize acomposite materialconstruction, such as a core made ofaluminumhoneycombor plasticized paper honeycomb, covered in a skin made of aluminum orcarbon fiber composite.Tail rotor blades can be made with both symmetrical and asymmetricalairfoilconstruction. The pitch change mechanism uses a cable control system or control tubes that run from the anti-torque pedals in thecockpitto a mechanism mounted on the tail rotor gearbox. In larger helicopters, the pitch change mechanism is augmented by a hydraulic power control servo. In the event of a hydraulic system failure, the mechanical system is still able to control the tail rotor pitch, though the control resistance felt by the pilot will be considerably greater.

The tail rotor is powered by the helicopter's main power plant, and rotates at a speed proportional to that of the main rotor. In both piston and turbine powered helicopters, the main rotor and the tail rotor are mechanically connected through afreewheeling clutch system,which allows the rotors to keep turning in the event of an engine failure by mechanically de-linking the engine from both the main and tail rotors. Duringautorotation,the momentum of the main rotor continues to power the tail rotor and allow directional control. To optimize its function for forward flight, the blades of a tail rotor have no twist to reduce the profile drag, because the tail rotor is mounted with its axis of rotation perpendicular to the direction of flight.

Reliability and safety

The tail rotor of aBell 206protected from ground strikes by a skid plat

The tail rotor and the systems that provide power and control for it are considered critically important for safe flight. As with many parts on a helicopter, the tail rotor, its transmission, and many parts in the drive system are oftenlife-limited,meaning they are arbitrarily replaced after a certain number of flight hours, regardless of condition. Between replacements, parts are subject to frequent inspections utilizing visual as well as chemical methods such asfluorescent penetrant inspectionto detect weak parts before they fail completely.

Despite the emphasis on reducing failures, they do occasionally occur, most often due to hard landings andtailstrikes,orforeign object damage.Though the tail rotor is considered essential for safe flight, the loss of tail rotor function does not necessarily result in a fatal crash. In cases where the failure occurs due to contact with the ground, the aircraft is already at low altitude, so the pilot may be able to reducecollective pitchof the main rotor and land the helicopter before it spins completely out of control. Should the tail rotor fail randomly during cruise flight, forward momentum will often provide some directional stability, as many helicopters are equipped with avertical stabilizer.The pilot would then be forced toautorotateand make an emergency landing with significant forward airspeed, which is known as arunning landingorroll-on landing.

The tail rotor itself is a hazard to ground crews working near a running helicopter. For this reason, tail rotors are painted with stripes of alternating colors to increase their visibility to ground crews while the tail rotor is spinning.

Alternative technologies

There have been three major alternative designs which attempt to solve the shortcomings of the tail rotor system.

The first is to use anenclosuredducted fanrather than openly exposedrotor blades.This design is referred to as afantailor "Fenestron",atrademarkofEurocopter(nowAirbus Helicopters) for itsDauphin-seriesutility helicopters.The enclosure around the fan reducestip vortex losses,shields the blades fromforeign object damage,protectsground crewsfrom potential hazard of an openly spinning rotor, and produces a much quieter and less turbulentnoiseprofile than a conventional tail rotor. The ducted fan uses more numerous shorter blades, but otherwise works in very similar thrust principles to a conventional tail rotor.

MD Helicopters 520N NOTAR

McDonnell Douglasdeveloped theNOTAR(NOTAilRotor) system, which eliminates having any rotating parts out in the open. The NOTAR system uses a variable pitch ducted fan driven by the helicopter's powerplant, but the ducted fan is mounted inside the fuselage ahead of the tail boom, and the exhaust passes through the tail boom to the end, where it is expelled out one side. This creates a boundary layer which causes the downwash from the main rotor to hug the tail boom according to theCoandă effect.This creates a force which cancels out the main rotor torque and provides directional control. The advantages of the system are similar to the Fenestron system discussed above.

There are at least four ways to eliminate the necessity of a tail rotor altogether:

• Tandem/transverse rotors:to use two non-overlapping main rotors which rotate in opposite directions, so that the torque created by one rotor cancels out the torque created by the other. Such configurations are commonly seen on heavy-lift helicopters like the tandem-rotoredCH-47 Chinook.

Thetiltrotordesign, as seen on theV-22 Osprey,is a variation of the transverse rotor design, where the rotors are installed in tiltablenacellesat the ends offixed wings.This allows the rotors to serve instead aspropellerswhen flying forward at full speed.

• Coaxial. Other designs such as theKamov Ka-50andSikorsky X2usecoaxial counter-rotating main rotors,which means that both rotors spin around the same axis but in opposite directions. The complexity of any dual main rotor system almost invariably requires the addition of afly-by-wireflight control system, which increases costs drastically.
• Intermeshing rotorsalso turn in opposite directions, but the blades rotate into the gaps between the opposing blades so the rotors can intersect each other's path without colliding. Invented byAnton Flettnerand used inFlettner Fl 282,Kaman HH-43 Huskie,andKaman K-MAX.
• Tip jet.Another way to eliminate the effect of torque created by the rotorwing is by mounting the engine on the tips of the rotorwing rather than inside the helicopter itself; this is called atip jet.One example of a helicopter using such a system is theNHI H-3 Kolibrie,which had a ramjet on each of the two wingtips, and an auxiliary power unit to spin up the rotor before starting the ramjets. Another example would be theFairey Rotodyne.Also, unpowered rotors used inautogyro,gyrodyne,and derived concepts do not need a tail rotor either, although nearly all models that utilize this concept of propulsion do need a second propeller in one way or another to drive them forward to begin with.

See also

• Piasecki X-49
• Loss of tail-rotor effectiveness